patch 8.2.0850: MS-Windows: exepath() works different from cmd.exe

Problem:    MS-Windows: exepath() works different from cmd.exe.
Solution:   Make exepath() work better on MS-Windows. (closes #6115)
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index e5c74e0..9b5d976 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -1187,6 +1187,30 @@
     call assert_equal(0, executable('notepad.exe.exe'))
     call assert_equal(0, executable('shell32.dll'))
     call assert_equal(0, executable('win.ini'))
+
+    " get "notepad" path and remove the leading drive and sep. (ex. 'C:\')
+    let notepadcmd = exepath('notepad.exe')
+    let driveroot = notepadcmd[:2]
+    let notepadcmd = notepadcmd[3:]
+    new
+    " check that the relative path works in /
+    execute 'lcd' driveroot
+    call assert_equal(1, executable(notepadcmd))
+    call assert_equal(driveroot .. notepadcmd, notepadcmd->exepath())
+    bwipe
+
+    " create "notepad.bat"
+    call mkdir('Xdir')
+    let notepadbat = fnamemodify('Xdir/notepad.bat', ':p')
+    call writefile([], notepadbat)
+    new
+    " check that the path and the pathext order is valid
+    lcd Xdir
+    let [pathext, $PATHEXT] = [$PATHEXT, '.com;.exe;.bat;.cmd']
+    call assert_equal(notepadbat, exepath('notepad'))
+    let $PATHEXT = pathext
+    bwipe
+    eval 'Xdir'->delete('rf')
   elseif has('unix')
     call assert_equal(1, 'cat'->executable())
     call assert_equal(0, executable('nodogshere'))